Measurements of a large variety of enzymes and other cellular components will be made in each of a representative series of rat tissues, including fetal, adult and neoplastic tissues. Special attention will be given to mammary gland during the stages of lactation, especially during involution of the gland, and its arginine and protein metabolism. The objective is to classify the different compositions of tissues as the products of the differently regulated genetic programs of gene expression, and to relate these to the functional differences among the tissues. This classification will also chemically define groups of related tissues, including neoplasms. The numerical interpretations will be done by principal components and multiple discrimination analyses.